This invention relates to the positioning and retention of a cylindrical object within a structure and, more particularly, to such positioning and retention when the cylindrical object is relatively fragile and may be damaged by an excessive inwardly directed point radial force.
A linear variable differential transformer (LVDT) is a sensitive instrument for measuring positional changes. The LVDT has a cylindrical LVDT body with a positional probe extending from the LVDT body along the cylindrical axis of the LVDT body. Small movements of the probe are detected by a transformer within the LVDT body. LVDTs are described in greater detail, for example, in U.S. Pat. No. 4,543,732.
The cylindrical body of the LVDT must be mounted to a stable support structure that holds the LVDT body in a fixed reference position from which the positional changes may be measured. In one approach to holding the LVDT body in a fixed position, the LVDT may be cast into a support, such as a block of epoxy. This approach has the drawbacks that the support may change dimensionally over time and with temperature, that the position of the LVDT body cannot be readily adjusted to a new fixed reference position, and also that the LVDT body cannot be readily removed from the support. In another approach, the LVDT body is placed into a cylindrical bore in the support and retained with a set screw. This approach allows the LVDT body to be repositioned along the bore, but there are significant drawbacks. It is often difficult to achieve the necessary mechanical tolerances on the bore. Additionally, the set screw must be tightened very precisely. If the set screw is undertightened, the LVDT body will wobble and become misaligned in the bore; if it is overtightened, the LVDT body may be deformed so that the LVDT body is damaged. Although the correct tightening of a set screw would seem to be a straightforward matter, in practice the realities of a production environment often result in operators using excessive force and devoting insufficient attention to this step. Additionally, even if the correct amount of force is applied, it is unevenly distributed and may result in damage to the sensitive LVDT.
There is a therefore a need for an improved approach for locating and clamping an LVDT body into a reference position. The present invention fulfills this need, and further provides related advantages.
The present invention provides an article in which a cylindrical LVDT body is supported. The LVDT body is precisely positioned both angularly and axially. Overtightening and undertightening of a set screw are avoided. The present approach is embodied in the application of most interest, the mounting of the LVDT body, but it is equally applicable to other situations where a cylindrical body must be precisely positioned both angularly and axially.
An article comprises a cylindrical object, and a support body having a keyed slot therethrough. In the application of most interest, the cylindrical object is the body of a linear variable differential transformer (LVDT). The keyed slot comprises a cylindrical aperture elongated along an aperture axis and a slot extending radially from the cylindrical aperture. A sleeve, preferably made of metal, is received in the keyed slot. The sleeve has an annular segment of a hollow cylinder received within the cylindrical aperture. The annular segment has a circumferential first annular end, a circumferential second annular end, an inner annular diameter DI sized to slidingly receive the cylindrical object therein, and an outer annular diameter DO sized for the annular segment to fit snugly (i.e., with a xe2x80x9cbearing fitxe2x80x9d) within the cylindrical aperture. The cylindrical object is received within the annular segment. The sleeve further has a first tab extending from the first annular end into the slot (and preferably not contacting a side of the slot), and a second tab extending from the second annular end into the slot and contacting a side of the slot. A force applicator, preferably a set screw, is positioned to controllably force the first tab toward the second tab to tighten the sleeve about the cylindrical object while retaining good alignment within the cylindrical aperture. The annular thickness of the sleeve is preferably small, so that it can readily flex. Desirably, (DOxe2x88x92DI) is about 10-15 percent of DO.
To mount a cylindrical object such as an LVDT body using the present approach, the keyed slot is first machined into the support body. The machining is preferably performed by electrodischarge wire machining (xe2x80x9cwire EDMxe2x80x9d), which produces good tolerances positionally, dimensionally, and angularly. The sleeve is machined, again preferably by wire EDM. The cylindrical object is placed into the sleeve, and the sleeve is inserted into the cylindrical aperture of the keyed slot with the tabs in the slot of the keyed slot. The cylindrical object is axially positioned by sliding it along the sleeve. When it is properly positioned, the set screw is tightened against the first tab to tighten the sleeve around the cylindrical object. The holding force of the set screw is thereby distributed around the circumference of the cylindrical object rather than being applied at a point, and overtightening is avoided by the spring deflection of the first tab.
The present approach thus achieves precise positioning of the cylindrical object without risk of overtightening a set screw that would potentially damage the cylindrical object. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.